Biological sensors found in nature have some of the best designs with incomprehensible features. Biomimetic sensor development involves learning various design features, sensing phenomenon, and material aspects from nature, and utilizing them to uniquely engineer or benefit man-made artificial sensors.
Knowledge obtained from the natural systems could significantly benefit the engineering of artificial devices. Bio-inspired studies try to look outside traditional domains into natural environmental processes to find key inspiration in order to result in novel designs for engineering systems. In the past, many researchers have shown substantial interest in developing bio-inspired systems, both in the macro- and micro-levels: for example piezoelectric inchworm motor inspired by the inchworms, flow sensors inspired by the cercal wind-receptor hair-like structures in crickets, materials capable of leg-less motion inspired by the locomotion of terrestrial limbless animals, mussel inspired adhesive materials, etc.
Blind cave fish are a unique fish species that are capable of swimming at high speeds in water without colliding with any underwater obstacles around them in spite of being blind. The blind cave fish accomplishes this surprising feat just by relying on arrays of flow and pressure-gradient sensors present on its body. An artificial analogue of similar arrays of flow and pressure sensors could greatly benefit underwater vehicles to visualize their surroundings and enable them to perform energy-efficient maneuvering. Individual biological sensors present on and under the skin of the blind cave fish are called neuromasts. These neuromasts consist of a gelatinous cupula with encapsulated cupular fibrils that support the soft cupular material that extends into the flow.
FIG. 1A shows a photograph 100 of a blind cave characin fish 102. In spite of being blind, the blind cave characin fish 102 displays an uncanny ability to swim at high speeds without collision with any underwater obstacles. It relies on two types of biological sensors present on and inside its skin to derive information about flows around its body called the superficial neuromasts and the canal neuromasts. In FIG. 1A, the dotted line 104 represents a lateral line of canal neuromasts on the body of the blind cave characin fish 102.
The canal neuromasts (CNs) are enclosed in fluid-filled canals present sub-dermally and are exposed to external water through pores in the skin of a blind cave characin fish. The body of the fish has more or less equally spaced CNs, each of which is located between two canal pores 112 on the over-enclosing canal as seen in the SEM images 110, 120 of FIGS. 1B and 1C.
The superficial neuromasts (SNs) are spatially distributed on the body of a blind cave characin fish and respond to the net movements between the fish and the surrounding water. Therefore, the SNs are responsible for flow velocity sensing and they respond slowly. FIG. 1D shows a schematic cross-sectional view of a superficial neuromast 130 of a blind cave characin fish, illustrating the sensing mechanism of the superficial neuromast 130. The morphology of individual superficial neuromasts 130 consists of bundles of haircells 132 encapsulated in a gelatinous cupula 134. External flow of water, as represented by 140, past the cupula 134 generates a frictional force on the cupula 134 causing the cupula 134 to bend and thereby the haircells 132 embedded inside the cupula 134 are stimulated. The cupular structure 134 acts as a mechanical coupler between surrounding water flow and the haircells 132 and increases the drag on the haircells 132 (or enhances the drag force exerted on the haircells 132) due to the increased surface area facing the flow. The cupula 134 consists of fibers called cupular fibrils 136 that extend from the base of the cupula 134 to its distal tip. The cupular fibrils 136 act as an internal structural support to the cupula 134, as a scaffold supporting the soft cupular material. The cupular fibrils 136 also allow the cupula 134 to grow much taller to reach beyond a boundary layer associated with the fish. The term “boundary layer” may refer to a layer of stationary or stagnant fluid in an immediate vicinity of a surface of the fish which may attenuate the velocity of fluid motion about the surface.
The canal neuromasts (CNs) are actuated only when there is a pressure difference between consecutive pores between those an individual CN is located in. The CNs therefore do not contribute to flow velocity sensing but perform acceleration sensing.
In the past, a few research groups worked towards developing a biomimetic hydrogel cupula to enhance the performance of a flow sensor. For example, flow sensors were developed with SU-8 hair-cells fabricated on thin silicon cantilever beams. A hydrogel cupula was formed by drop-casting polyethylene glycol (PEG) polymer on the SU-8 haircells. However, developing high-aspect ratio pillars by SU-8 processing is a very cumbersome process. There may also be issues related to the shape of very tall SU-8 pillars.